Boko Haram is Nigeria’s biggest Islamist group. It has grown increasingly violent under the stewardship of new leader Abubakar Shekau. Find out more with our video guide

The Islamic State welcomed a pledge of allegiance made to it by Boko Haram and vowed to press with its expansion, according to a recording on Thursday purportedly from its spokesman.

“We announce to you to the good news of the expansion of the caliphate to west Africa because the caliph … has accepted the allegiance of our brothers of the Sunni group for preaching and the jihad,” Isis spokesman Abu Muhammad al-Adnani said in the message, using the Arabic name for Nigeria’s Boko Haram extremist group.

In the tape of some 30 minutes, Adnani urges Muslims to join militants in west Africa and played down “victories” by the US-led coalition and Iraqi forces against his group.

Boko Haram-Isis alliance is nothing but superficial propaganda

Read more

The radical Sunni movement has seized large swathes of Iraq and Syria and declared an Islamic “caliphate” there. It has also drawn expressions of allegiance from jihadists in Egypt and Libya.

A combination of army, police and volunteer forces moved into northern and southern Tikrit, the hometown of former Iraqi president Saddam Hussein and a main Isis stronghold, after making major gains around the city.

But Adnani said Isis “continues to resist … and the victories proclaimed by the coalition are only illusions”, claiming that only “inches of land” in Iraq had been recaptured by loyalist forces.

And he insisted that the group is “sure of its victory” regardless of the challenges it is facing.

“God is on our side and gives us the strength to combat this armada of Crusader countries,” he said.

For months, Isis has been targeted with air strikes from a coalition led by the United States, and already suffered territorial setbacks in Syria and Iraq.